1. Field of the Invention
The present invention relates to a fuel cell system. Specifically, the present invention relates to a fuel cell system in which the total replacing amount of gas during the OCV purge increases during low temperature startup, as compared with during normal startup.
2. Related Art
In recent years, fuel cell systems have gained the spotlight as a new power source for automobiles. For example, a fuel cell system is provided with a fuel cell producing electric power from chemical reactions of reactive gas, a reactive gas supply device supplying the reactive gas to the fuel cell through a reactive gas channel, and a control device controlling the reactive gas supply device.
For example, the fuel cell consists of a plurality, e.g., tens or hundreds, of stacked cells. Each of the cells consists of a pair of separators sandwiching a membrane electrode assembly (MEA) which consists of two electrodes, an anode (positive electrode) and a cathode (negative electrode), and a solid polymer electrolyte membrane sandwiched by these electrodes.
Supplying anode gas to the anode and cathode gas to the cathode causes an electrochemical reaction by which the fuel cell generates electric power. For example, hydrogen gas and air containing oxygen are used for the anode gas and the cathode gas, respectively. Since only water, which is essentially harmless to the environment, is produced during electric power generation, the fuel cell has garnered attention from the viewpoint of environmental impact and usage efficiency.
In such a fuel cell system, a condition in which electric power generation is stopped by stopping the supply of anode gas and cathode gas causes a differential pressure between both electrodes, so that impurities contained in the cathode gas diffuse to the anode side, whereby the concentration of anode gas in the anode channel decreases. Accordingly, when the fuel cell system starts, so-called OCV purge processing, in which additional anode gas is supplied to replace gas retained in the anode electrode side of the fuel cell, is conducted in order to ensure power generation performance of the fuel cell.
However, residual water in the fuel cell may freeze at a membrane electrode, whereby the effective power generation area decreases upon low temperature start. This could lead to difficulty in starting the fuel cell system. Thus, Unexamined Japanese Patent Application, First Publication No. 2005-44795 discloses a fuel cell system in which the supply pressure and the flow of anode gas during OCV purge processing increase upon low temperature start, as compared with upon normal temperature start. The supply pressure and the flow of anode gas increases as described above, so that the power generation performance of the fuel cell can be ensured to be prompt by effectively using a smaller reactive area than that upon normal temperature start.